Presently, the Space Shuttle Orbiters derive electrical energy from "fuel cells" which chemically combine oxygen and hydrogen to produce electricity. Fours sets of cryogenic storage tanks containing the oxygen and hydrogen reactants are normally expended in about eight days at typical power usage levels.
To extend the duration of an Orbiter mission for more than eight days, it is possible to utilize additional storage tanks. Indeed, Columbia (OV102) sometimes carries a fifth tank set to extend the mission by about two days. For longer durations, it becomes necessary to carry even larger amounts of reactants for the fuel cells in the payload bay (approximately 400 lbm/day of flight extension). An additional pallet of four tank sets could be used to increase the mission duration from 8 to 16 days. The pallet containing the four tank sets, however, adds a mass of about 9000 lb which reduces the weight of the payload that can be lifted into orbit. The weight of the storage tank sets are such that extended durations beyond two weeks may not be practical due to the weight of the storage tanks required for such a mission.
Furthermore, the combination of these reactants contained in the storage tanks is a highly combustible mixture. A significant safety concern is thus raised whenever large quantities of the reactants are carried in the payload bay, especially during launch.
An earlier conceptual design of providing the Orbiter with electricity derived from solar energy was called a Power Extension Package (PEP). The PEP featured a solar array package and was carried into space on a pallet in the Orbiter payload bay. The "Remote Manipulator System" (RMS) was then used to grapple the array, lift it out of the payload bay and allow the array to unfold. The array had articulating joints permitting the Orbiter to assume any arbitrary attitude while the PEP remained pointing at the sun.
There are several disadvantages inherent in this PEP system. It requires the RMS to lift the PEP out into space and to maintain it perpendicular to the sun. To achieve the perpendicular relationship, three articulation joints had to be adjusted and readjusted upon any shift in Orbiter attitude. Power transfer took place across all of the rotating joints, which in turn necessitated complicated wiring systems. The required use of the RMS, which is not carried on most missions, added weight to the Orbiter and complexity to the mission. These factors had a very significant impact on mission cost.
In addition, PEP motion could subject the Orbiter to acceleration forces which would degrade microgravity conditions. This had the potential effect of interfering with microgravity experiments or manufacturing processes being conducted.
U.S. Pat. No. 4,630,791 illustrates a space-based solar operated power module which also relies on the RMS to deploy, meaning that it also produced many of the same disadvantages as the PEP.
Moreover, the extension of the solar panel mounting tubes in U.S. Pat. No. 4,630,791 longitudinally along the length of the Orbiter's payload bay has a highly limiting effect on the size of the cargo which can be stowed. The stowing of the solar panel mounting tubes longitudinally along the payload bay can also lead to problems of entanglement between the mounting tubes and various cargo structures stowed in the cargo bay.